Actuator materials and mechanisms that convert electrical, chemical, thermal, or photonic energy to mechanical energy have been sought for over a century. Nevertheless, humankind has had little success in replicating the wondrous properties of natural muscle, which has meant that the most advanced prosthetic limbs, exoskeletons, and humanoid robots lack critically needed capabilities.
Probably no other material has been described for so many fundamentally different types of actuators than carbon nanotubes. Demonstrated electrically powered and fuel powered nanotube actuators provide up to a few percent actuator stroke and a hundred times higher stress generation than natural muscle. Large stroke pneumatic nanotube actuators have been demonstrated that use electrochemical gas generation within nanotube sheets. In other studies, nanotubes have been used either as electrodes or as additives to profoundly modify the response of other actuating materials—like dielectric, ionically conducting, photoresponsive, shape memory, and liquid crystal polymers.
The following provide examples of these diverse types of actuators based on carbon nanotubes. Electrostatic attraction and repulsion between two nanotubes was used for cantilever-based nano-tweezers [P. Kim, C. M. Lieber, Science 286, 2148-2150 (1999)] and mechanically-based switches and logic elements [T. Rueckes, K. Kim, E. Joselevich, G. Y. Tseng, C.-L. Cheung, C. M. Lieber, Science 289, 94-97 (2000); V. V. Deshpande, H.-Y. Chiu, H. W. Ch. Postma, C. Mikó, L. Forró, M. Bockrath, Nano Letters 6, 1092-1095 (2006)]. On the macroscale, electrically powered [R. H. Baughman et al., Science 284, 1340-1344 (1999); U. Vohrer, I. Kolaric, M. H. Hague, S. Roth, U. Detlaff-Weglikowska, Carbon 42, 1159-1162 (2004); S. Gupta, M. Hughes, A. H. Windle, J. Robertson, J. Appl. Phys. 95, 2038-2042 (2004)] and fuel powered [V. H. Ebron et al., Science 311, 1580-1583 (2006)] carbon nanotube actuators provided up to a few percent actuator stroke and a hundred times higher stress generation than natural muscle. Demonstrated large stroke pneumatic nanotube actuators used electrochemical gas generation within nanotube sheets [G. M. Spinks et al., Advanced Materials 14, 1728-1732 (2002)]. Carbon nanotube composites with organic polymers provided photoresponsive [S. V. Ahir, E. M. Terentjev, Nature Materials 4, 491-495 (2005)], shape memory [H. Koerner, G. Price, N. A. Pearce, M. Alexander, R. A. Vaia, Nature Materials 3, 115-120 (2004)], and electromechanical [S. Courty, J. Mine, A. R. Tajbakhsh, E. M. Terentjev, Europhysics Letts. 64, 654-660 (2003)] actuators. Previous work has also demonstrated the use of polymer-filled non-twisted carbon nanotube yarns as thermally powered shape memory materials, but reversible actuation was not achieved [P. Miaudet et al., Science 318, 1294-1296 (2007)]. In other work, dispersed carbon nanotubes or nanotube sheets have been used for electrically heating thermally actuating materials to provide cantilever deflections [A. T. Sellinger, D. H. Wang, L.-S. Tan, R. A. Vaia, Adv. Mater. 22, 3430 (2010); L. Chen, C, Liu, K. Liu, C. Meng, C. Hu, J. Wang, S. Fan, ACS Nano 5, 1588 (2011); and Y. Hu, W. Chen, L. H. Lu, J. H. Liu, C. R. Chang, ACS Nano 4, 3498-3502 (2010)]. Major limitations exist for the above described carbon nanotube artificial muscles, as well as prior art artificial muscles of any type. These limitations include slow response, low stroke or force generation, short cycle life, hysteresis in actuator response, use of electrolytes, or a narrow temperature range for operation—and in most cases a combination of some of these and other limitations (like low energy conversion efficiency).
Artificial muscles based on carbon nanotube artificial aerogel sheets have been developed that can operate at extreme temperatures (near 0 K to above 1900 K) where prior-art muscles cannot operate. They provide stroke rates and strokes that can exceed 4×104%/s and 250% in one direction and generate over 30 times higher force than for the same weight and length natural muscle [A. E. Aliev et al., Science 323, 1575-1578 (2009) and A. E. Aliev et al., PCT International Appl. WO 2010/019942 A2 (2010)]. Unfortunately, these carbon nanotube muscles typically use thousands of volts of applied potential and cannot be scaled in the thickness direction to provide muscles that can support heavy loads.
Electrochemically powered multiwalled carbon nanotube (MWNT) yarn muscles [J. Foroughi et al., Science 334, 494-497 (2011)] can generate over a thousand times larger rotation per length than previous torsional muscles based on shape memory alloys [A. C. Keefe, G. P. Carman, Smart Mater. Struct. 9, 665-672 (2000)], ferroelectric ceramics [J. Kim, B. Kang, Smart Mater. Struct. 10, 750-757 (2001)] or conducting polymers [Y. Fang, T. J. Pence, X. Tan, IEEE/ASME Trans. Mechatronics 16, 656-664 (2011)]. The twist-spun actuating yarn can accelerate a paddle to 590 revolutions/minute in 1.2 s [J. Foroughi et al., Science 334, 494-497 (2011)] and provide similar torque and mechanical power generation per yarn weight as the gravimetric capabilities of large electric motors. However, these advantages come at a cost. Since actuation arises from yarn volume changes generated by ion influx during electrochemical double-layer charge injection, overall system gravimetric performance is degraded by the need for electrolyte, counter electrode, and device packaging, which add much more to actuator weight than the actuating yarn. The liquid electrolyte also limits operating temperature and voltage, as well as actuation rate and deployment possibilities.
In some invention embodiments, the present invention eliminates the need for electrolyte, counter electrode, and special packaging by using a solid guest material in the yarn to generate the volume changes that produce tensile and torsional actuation. As used herein, the term “tensile actuation” denotes actuation in the length direction of an actuator, regardless of whether the actuator elongates or contracts in the length direction during an actuation step. In hybrid nanotube muscles the twist-spun nanotubes confine this actuating guest in both solid and molten states, and provide the mechanical strength and helical geometry enabling torsional actuation and enhanced tensile actuation. Yarn actuator structure will be engineered to maximize either torsional or tensile actuation. Reversible actuation will be powered electrically, photonically, or chemically.
Furthermore, with embodiments of the present invention, the Applicant has provided demonstration of high-cycle-life, large-stroke, and high-rate torsional and tensile artificial muscles that:                (1) Comprise only a neat or hybrid twist-spun nanotube yarn as the actuating element.        (2) Require no electrolyte or counter-electrodes and operate at low voltages.        (3) Can be electrically, chemically, and photonically powered.        (4) Deliver over two million reversible torsional actuation cycles, wherein a hybrid yarn muscle spins a rotor at an average 11,500 revolutions/minute. This rotation rate is 20 times higher than we previously demonstrated for electrochemical carbon nanotube muscles and over 20,000 times higher than for previous muscles based on shape memory alloys, ferroelectric ceramics, or conducting polymers.        (5) Generates a gravimetric torque per muscle weight that is (a) five times higher than for previous electrochemical torsional muscles and (b) slightly higher than for large electric motors.        (6) Delivers 3% tensile contraction at 1,200 cycles/minute for over 1.4 million cycles.        (7) Delivers 27.9 kW/kg average power density during muscle contraction, which is 85 times higher than for natural skeletal muscle. Including times for both actuation and reversal of actuation, a contractile power density of 4.2 kW/kg was demonstrated, which is four times the power-to-weight ratio of common internal combustion engines.        (8) Demonstrated a maximum tensile contraction of 10%.        (9) While the above demonstrations of (3)-(8) are for hybrid muscles in which a twist-spun nanotube host confines a volume expanding guest, the Applicant of the present invention has also demonstrated torsional and tensile actuation for neat twist-spun nanotube yarns that are electrothermally heated to incandescent temperatures. These neat muscles provide 7.3% tensile contraction while lifting heavy loads at extreme temperatures where no other high work capacity actuator can survive.        (10) Demonstrations include torsional motors, contractile muscles, and sensors that capture the energy of the sensing process to mechanically actuate.        
Complex coiled fiber geometries are used to dramatically increase actuator performance compared with that of prior art nanofiber yarn muscles.
Paraffin waxes are used in some invention embodiments as prototypical guests in carbon nanotube yarns because of high thermal stability; the tunability of transition widths and temperatures; the large volume changes associated with phase transitions and thermal expansion; and their ability to wet carbon nanotubes. Such waxes have been long investigated and commercially deployed as thermally or electro-thermally powered actuators [E. T. Carlen, C. H. Mastrangelo, Journal of Microelectromech. Syst. 11, 165 (2002)]. By confining the actuating wax in the nanosized pores of a carbon nanotube yarn, Applicant has avoided conventional hydraulic and external heating systems and directly use a muscle-like geometry, where high surface/volume and thermal and electrical conductivities enhance response rate and a helical geometry enables both torsional rotation and tensile contraction.
In some other invention embodiments, twist insertion and optional fiber coiling is applied to ordinary polymer fibers, like the high strength polyethylene and nylon used for fishing line and sewing thread, in order to obtain high performance artificial muscles that provide torsional actuation, tensile actuation, or a combination thereof. Like for nanofiber yarn invention embodiments, (1) the need for electrolyte, counter electrode, and special packaging is also eliminated, since electrochemical processes are not required for actuation and (2) reversible actuation can be powered electrically, photonically, thermally, or chemically for the twisted and for the coiled polymer fibers.
Both cost and performance provide major advantages for the twisted and coiled polymer fibers. While wires of shape memory metals can generate giant stresses and large strokes and provide fast contractions during electrothermal actuation, these artificial muscles are very expensive—popular high-performance NiTi wires cost about $1400/pound and $1.50/m. In contrast, commercially available polymer fibers that are precursor to the polymer muscles are inexpensive (typically ˜$2.50/pound), and the processes needed to convert the commercially fibers to artificial muscles (twist insertion and optional incorporation of conductor) are inexpensive.
Also, competing shape memory metal actuators are heavy and provide hysteretic actuation, which makes them difficult to precisely control, since actuation depends upon prior history within a cycle even when the applied load is constant. Thermally powered shape memory polymer fibers and polymer-filled, non-twisted carbon nanotube fibers can deliver giant strokes and contractile work capacities [P. Miaudet et al., Science 318, 1294-1296 (2007)], but provide largely irreversible actuation. Electrochemically driven fibers of organic conducting polymers also provide large strokes, but have poor cyclability and require an electrolyte containment system, which adds to system weight and cost. Invention embodiments will eliminate all of these problems.